Ideally the trip to the south-West would allow Phelan to give his first-team men a rest before their clash against Manchester United on Saturday.
But he knows there is no question of being able to do so with so few resources to select from.
"Everybody's going to have to go to Exeter," he said. "It's important, as we're in a bit of a groove, that they all keep this momentum of togetherness. At this moment of time everyone's happy with the performances and they all want to play.
"We have to realistic, we have younger players who really fancy their chances. They want to get in to a winning team. So there will be a couple of changes, we can't take any risks but overall they want to carry on and we know we have not got the luxury of rotating.
"We know it's going to be tough - you can't take anything for granted in this game. They will see us as a scalp and Exeter have not done too bad at all in the cups in the last few years."